High resolution mass spectrometry is widely used in the detection and identification of molecular structures and the study of chemical and physical processes. A variety of different techniques are known for the generation of a mass spectrum using various trapping and detection methods. Once such technique is Fourier Transform Ion Cyclotron Resonance (FTICR). FTICR's use the principle of a cyclotron, wherein a high frequency voltage excites ions to move in a spiral within an ICR cell. The ions in the cell orbit as coherent bunches along the same radial paths but at different frequencies. The frequency of the circular motion is inversely proportional to the ion mass.
The coupling of a linear ion trap with a Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer was introduced to separate the ion production region from the ion detection region of the FTICR. The combination of the two ion traps provides a powerful combination which can produce high sensitivity, high mass accuracy, and high resolution in an easy to use package. One undesirable aspect of this combination however is the problems associated with transferring ions from the linear ion trap to the FTICR cell. Due to vacuum requirements and the location of the FTICR cell in the center of a superconducting magnet, the ion transfer distance is typically a meter or more.
Ions are normally released from the linear trap with a fixed amount of kinetic energy (˜1V), and the DC offset of all ion optics are held static during the transfer. This means that the velocity of an ion will be mass-to-charge (m/z) dependent, and in a MS mode of operation, only ions having substantially the same mass to charge ratio or ions having a relatively narrow range of mass to charge ratios will enter the FTICR at substantially the same time. The gated trapping mechanism most commonly used for FTICR is able to only catch a ˜100 microsecond window of ions, which leads to transfer time dependent ion abundances. With short transfer times, low m/z ions are favored, while at long transfer times high m/z ions are favored. Higher energy ions arrive at the detector ahead of lower energy ions having the same mass. This spreading of flight times limits the mass range of the spectrometer.
It is desired to provide an improved mass spectrometer and in particular a mass spectrometer which enables ions with a wide range of m/z values to be trapped in a FTICR without compromising the results attained from use of such a mass spectrometer.